Abstract

Rock-avalanche deposits can be significant components of aggradational sequences formed in dry climates. Features distinguishing such deposits from their closest counterparts, debris-flow deposits, reflect the higher energies and shear strengths associated with rock-avalanche lobes, and their tendency to generate their own matrix through pulverization associated with the failure event.

Evaluation of sedimentary breccias in Oligocene-Miocene strata deposited under dry climatic conditions in west-central Arizona indicates that many are of rock-avalanche origin. Some are sufficiently large to have represented large rock avalanches (that is, sturzstroms) at the time of emplacement and display features consistent with descriptions of such lobes. Some rock-avalanche deposits interbedded with lacustrine sediments represent initially subaerial lobes that flowed into lakes. These bodies locally are intruded by substrate-derived injection structures and contaminated by lakebed mud; mud contamination was initially concentrated along the bases of lobes but affected a progressively greater proportion of the flows with increasing subaqueous runout. Contaminated portions of rock-avalanche lobes exhibit features consistent with decreased shear strength, and thoroughly contaminated lobes appear to have transformed into slow-moving, slurry-like flows that experienced internal cycling of debris.

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